RADIOACTIVITY

IGCSE Physics: Radioactivity
Chapter 20 Study Guide

1. Detection of Radioactivity

Radioactivity is the spontaneous random emission of ionizing radiation from unstable atomic nuclei. We cannot see it, but we can detect it.

Background Radiation

We are constantly exposed to low-level radiation called Background Radiation. It comes from natural and artificial sources:

  • Natural: Radon gas (air), Rocks, Cosmic rays (from space), Food (e.g., Bananas).
  • Artificial: Medical X-rays, Nuclear power waste.

Corrected Count Rate Formula

To measure the radiation from a specific source, you must subtract the background radiation from your total reading.

True Rate = Measured Rate - Background Rate

Example: If your detector reads 100 counts/min, and the background is 20 counts/min, the True Rate is 80 counts/min.

2. The Three Types of Emission

Unstable nuclei emit three main types of radiation: Alpha (α), Beta (β), and Gamma (γ).

Properties Table

Type Symbol What is it? Penetration Ionizing Power
Alpha α / He Helium Nucleus (2p, 2n) Low (Paper) High
Beta β / e Fast Electron Medium (Aluminium) Medium
Gamma γ EM Wave (Energy) High (Lead) Low

Penetration Power Visualization

Source Paper Aluminium (3mm) Lead α β γ

Behavior in Electric Fields

  • Alpha (+): Attracted to Negative plate. Deflects slightly (heavy).
  • Beta (-): Attracted to Positive plate. Deflects sharply (light).
  • Gamma (0): Goes straight. No charge.

3. Radioactive Decay Equations

When a nucleus decays, it changes into a new element. We must balance the Nucleon number (top) and Proton number (bottom).

Alpha Decay (α)

Top number decreases by 4. Bottom number decreases by 2.

AZX → A-4Z-2Y + 42He

Beta Decay (β)

Neutron turns into proton + electron. Top number stays same. Bottom number increases by 1.

AZX → AZ+1Y + 0-1e

Gamma Emission (γ)

Release of energy. No numbers change.

AZX → AZX + γ

4. Half-Life (The Clock)

Definition: The time taken for half the nuclei of an isotope in a sample to decay.

The Decay Curve

To find half-life from a graph:

  1. Find the starting count rate (e.g., 80).
  2. Divide by 2 (e.g., 40).
  3. Find the time on the x-axis corresponding to that value.
Time Count Rate Start (N) N / 2 Half-Life

5. Safety & Uses

Safety Precautions

  • Time: Reduce exposure time.
  • Distance: Use tongs, stand far away.
  • Shielding: Use lead boxes or protective suits.

Common Uses

Use Type Reason
Smoke Detectors Alpha Ionizes air; stopped by smoke. Long half-life.
Thickness Control Beta Passes through paper/foil but amount changes with thickness.
Medical Tracers Gamma Escapes the body to be detected. Short half-life.
Checking Pipes Gamma Penetrates thick metal to find cracks.
Based on IGCSE Physics Chapter 20 Sources

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